The present invention relates to a method for dissolving slightly soluble sludge which is produced during chemical conversion coating of metals and for recovering constituents such as phosphoric acid and metals from the sludge to reuse.
In order to form and fix non-corrosive insoluble phosphate on metal surface for the purposes such as rust inhibition, electric insulation and undercoating for painting or lubrication, chemical conversion coating with phosphoric acid and metal phosphate is widely used, both domestic and abroad. The surface treatment of metal is applied to various articles such as an automobile body, household electrical appliances, a cannon ball, an iron pipe, building materials, and etc.
The solvents used in the phosphate chemical conversion coating as described above are sort out zinc type, manganese type, and iron type solvents depending on the type of the metal treated. For example, a solution composed of zinc dihydrogenphosphate and phosphoric acid is primarily used, and the chemical coating is conducted by spray or immersion method. For instance, when a car body made of steel plate is immersed with a zinc phosphate type agent and coating is formed on surface of the car body, crystalline of Hopeit [Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O] or phosphorite in which some zinc substituted with iron is deposited. Iron in the body reacts with phosphoric acid in an etching agent and large quantity of sludge containing iron phosphate (III) as main component is produced in the bath as shown in the following reactions, EQU Fe+2H.sub.3 PO.sub.4 .fwdarw.Fe(H.sub.2 PO.sub.4).sub.2 +H.sub.2 EQU 3Zn(H.sub.2 PO.sub.4).sub.2 .fwdarw.Zn.sub.3 (PO.sub.4).sub.2 +4H.sub.3 PO.sub.4.
At present, however, no method is available to re-utilize large quantity of sludge thus produced, and the sludge must be dumped and buried in specific area or abandoned into the sea at considerable expense or, for example, must be treated with alkali under water and heating (Japanese Patent Publication 58-39768) or treated by hydrothermal reaction at high temperature and pressure. However, because of difficulty in operation and high cost for the equipment and devices associated with such operation, these methods and techniques are still far from practical applications such as complete dissolution of sludge and effective utilization of resources in terms of workability and economic feasibility.
The enforcement of strict legal regulations on the disposal of industrial waste in recent years has now made it impossible to execute conventional methods for waste disposal, and there are now strong and imminent demand on the development of methods, which are simple in operation and high in workability and require less cost and which are useful for chemical dissolution of the sludge and effective utilization of resources and for protection of global environment.